Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $19.81   7.866%   $21.3683   $21.37   $21.35   $21.40  
  Purchase #2 $57.81   6.374%   $61.4948   $61.49   $61.50   $61.50  
  Purchase #3 $106.37   8.499%   $115.4104   $115.41   $115.40   $115.40  
  Purchase #4 $62.61   8.618%   $68.0057   $68.01   $68.00   $68.00  
  Purchase #5 $42.81   9.298%   $46.7905   $46.79   $46.80   $46.80  
  Purchase #6 $18.93   3.438%   $19.5808   $19.58   $19.60   $19.60  
  Purchase #7 $213.76   9.212%   $233.4516   $233.45   $233.45   $233.50  
  7 purchase totals = $522.10       $566.1021   $566.10   $566.10   $566.20